One of the most recent releases of the Mac OS called Sierra has the option to install from anywhere "hidden" in the Mac Settings of Security & Privacy.
This at first blush looks like an attempt for Apple to try to force more software developers to pay them a percentage of sales by distributing through the store. But I'm sure they will position it as "maintaining security" on Mac computers.
HOWEVER, there is a workaround but it is NOT OBVIOUS and clients will have to be told about this. I will also add this to the Mac downloads page.
The trick is to hold the Control button down on the keyboard when clicking the .pkg installer file.
And then clicking Open from the dialog. It may also require that you enter a password at that point.
Control-click the app > Open. Click Open on the dialog that appears.
NOTE: There are not any viruses in our software. It is checked at multiple stages of the process before being uploaded online.